1. Field of the Invention
The present invention relates to a cordless telephone system which comprises a connection unit connected to a wire telephone circuit and a wireless or radio telephone set connected through a radio communication line to the connection unit and, more particularly, to a cordless telephone system which allows extension speech or communication between a wire telephone set connected to the connection unit and a wireless telephone set.
2. Description of the Related Art
A conventional cordless telephone system, as shown in FIG. 4, comprises a connection unit 1 and a radio telephone set 2. A wire telephone set 3 is connected if necessary to connection unit 1 through such a connector as a rosette to allow the communiation of a wire telephone circuit 4 with the wire or wireless telephone set 3 or 2.
The radio telephone set 2 is connected through a radio communication circuit to the connection unit 1 and the operation thereof will be summarized in the following.
First, a signal sent from the wire telephone circuit 4 is supplied through a hybrid circuit 5 and a base band circuit 6 to a radio frequency unit 7 as its modulation oinput. The hybrid circuit 5 contains a line switch for DC loop formation.
Radio waves modulated at the radio frequency unit 7 are transmitted from an antenna 8 to the radio telephone set 2.
Radio waves emitted from an antenna 9 of the radio telephone set 2, on the other hand, are received by the antenna 8 of the connection unit 1 to be demodulated at the radio frequency unit 7. A demodulated signal is sent to the wire telephone circuit 4 through the base band circuit 6 and the hybrid circuit 5. During the above operation, a control circuit 10 incorporated in the connection unit 1 performs radio interconnection control between the connection unit 1 and the radio telephone set 2 as well as radio channel control of the radio frequency unit 7.
The radio telephone set 2, which, like the connection unit 1, includes the antenna 9, a base band circuit 11, a radio frequency unit 12 and a control circuit 13, is arranged so that a signal received at the antenna 9 is sounded from a receiver of a handset of the telephone set 2. A user's voice inputted from a transmitter 15 of the handset of the telephone set 2 is sent to the base band circuit 11 and then to the radio frequency unit 12 as its modulation input to be transmitted from the antenna 9. A loud speaker 16 acts as a sounder which generates a ringing tone when the telephone set 2 receives a call transmitted from the wire telephone circuit 4. When a 16-Hz detecting circuit 18 of the connection unit 1 detects the ringing signal of 16 Hz sent from the wire telephone circuit 4, this causes the control circuit 10 to transmit the received call signal from the antenna 8. The radio telephone set 2, when receiving the received call signal at the antenna 9, transmits an answer signal to the received call signal, from the antenna 9 back to the connection unit 1, and at the same time the control circuit 13 orders the loud speaker 16 to generate a ringing tone.
If the antenna 8 of the connection unit 1 fails to catch the received-call answer signal from the radio telephone set 2, then the connection unit 1 judges that the radio telephone set 2 has failed in the connection of the received call and orders a loud speaker 17 incorporated in the connection unit 1 to generate a ringing tone, prompting the user to answer the wire telephone set 3.
When the radio telephone set 2 is used at a location relatively apart from the connection unit 1, it is often desired to perform an extension communication between the wire and wireless telephone sets 3 and 2.
Such a conventional cordless telephone system as described above, in which the wire telephone set 3 is connected merely in parallel to the connection unit 1, has had a problem that the system permits only three-party communication including a party at the end of the wire telephone circuit 4 through an exchange (not shown). In addition, the telephone system has also been defective in that, during outside-line communication of one of the wire and wireless telephone sets 3 and 2 with a party at the other side of the exchange, if a user of the other telephone set picks up the handset to put the phone in its off-hook state, then the user of the other telephone set can listen in or wiretap the speech through the handset.